Mortality and Severe Obesity: Curiously Low Mortality Among the Heaviest NHANES III Participants

• Activity Number: 589
• Type: Contributed
• Date/Time: Wednesday, August 3, 2011 : 2:00 PM to 3:50 PM
• Sponsor: ENAR
• Author(s): Scott Keith*+ and Kevin Fontaine and David B. Allison
• Companies: Thomas Jefferson University and The Johns Hopkins University and University of Alabama at Birmingham
• Keywords: survival analysis ; obesity ; mortality ; BMI ; selection bias

Abstract

The Third National Health and Nutrition Examination Survey (NHANES III; sampled from 1988-1992) has been used to estimate the relationship between measured body mass index (BMI; weight in kilograms/squared height in meters) and mortality in the United States. Although BMI random variables are ratios of continuous quantities, their values are commonly categorized for analysis as underweight (< 18.5), normal weight (18.5-24.9), overweight (25.0-29.9), obese (30.0-34.9), or severely obese (=35.0). While the other BMI categories represent ranges of ~5 BMI units, the severely obese represents 30 units or more. We used various functional bases and BMI categorizations in Cox proportional hazards models, as well as other analytical tools, to examine and contrast mortality rates among the severely obese adults of NHANES III. Preliminary findings suggest that, compared to normal weight, subgroups of the severely obese have surprisingly low mortality rates through 2000 (e.g., HR=1.11 95% CI: 0.69, 1.77 for women with 39=BMI< 45; HR=0.30 95% CI: 0.12, 0.76 for men with BMI>41). This finding could be due to a number of factors and we hypothesize a peculiar form of selection bias.